1. Field of the Invention
The present invention relates to an oscillator circuit and an L load differential circuit, and particularly to an oscillator circuit using an LC resonant circuit as well as an L load differential circuit mountable on the oscillator circuit.
2. Description of the Background Art
In wireless devices such as a cellular phone, a local oscillator circuit is used for frequency conversion of received signals into low-frequency signals allowing demodulation and for frequency conversion of send signals (i.e., signals to be sent) into high-frequency signals, and is required to have a wide oscillation frequency range and can lower noises (phase noises) at and around an oscillation frequency.
A Voltage Control Oscillator (VCO), which is a kind of local oscillator circuit, utilizes an oscillation phenomenon caused by positive feedback of the circuit, and can control the oscillation frequency by a control signal. In general, the VCO employs a resonant circuit or utilizes a delay time of a circuit.
In connection with the VCO utilizing the resonant circuit, a negative conductance LC oscillator circuit is known as an oscillator circuit utilizing negative resistance characteristics of a positive feedback circuit formed of transistors, as disclosed, e.g., in A Yamagishi et al., “A Low-Voltage 6-GHz-Band CMOS Monolithic LC-Tank VCO Using a Tuning-Range Switching Technique”, IEICE Trans. Fundamentals, vol. E84-A, No. 2, February 2001. Since this oscillator circuit uses the LC resonant circuit including an inductor element and a capacitor element, it can achieve good phase noise characteristics, and application to VCOs for portable cordless devices has been expected.
A structure and an operation of a conventional VCO will now be described in connection with, e.g., a negative conductance LC oscillator circuit.
A conventional VCO is formed of an LC resonant circuit formed of two inductor elements and two diode elements, and a positive feedback circuit formed of two transistors each having a gate connected to a drain of the other.
In this structure, an input impedance Rin of the positive feedback circuit is equal to −2/gm (Rin=−2/gm) where gm represents a mutual conductance of each transistor. Therefore, the VCO oscillates when an absolute value |Rin| of the input impedance is equal to or lower than an equivalent parallel resistance of the resonant circuit. Assuming that inductances L1 and L2 of the two inductor elements are both equal to L (i.e., L1=L2=L) and a variable junction capacitance is equal to Cvar, an oscillation frequency fosc is expressed by the following formula (1):
                              f          osc                =                  1                      2            ⁢            π            ⁢                                          LC                var                                                                        (        1        )            
Accordingly, oscillation frequency fosc can be controlled in accordance with junction capacitance Cvar varied by the control voltage connected to the diode element.
An oscillation amplitude Aosc of the VCO is expressed by the following formula (2), and takes the value proportional to oscillation frequency fosc.Aosc∝2πfoscL  (2)
When the LC resonant circuit included in the VCO having the above differential structure is to be used for 1 to 2 GHz, an LC type using a lumped constant is predominantly employed because it can reduce an area of an integrated structure. A variable capacitance (varactor diode) is predominantly used as the capacity element. The inductor element is formed of a spiral inductance, which is formed of a spiral interconnection and a leader interconnection, and is generally formed on the same substrate as the transistor elements.
Accordingly, the inductance of the inductor element is uniquely determined in accordance with the form of the spiral, and cannot be adjusted unless a mask design is changed.
Meanwhile, the transistor elements formed on the same substrate do not necessarily exhibit designed characteristics due to variations in manufacturing steps. Therefore, inductance mismatching occurs between the inductor elements, which reduces yield.
Recently, many kinds of inductance-variable elements, of which inductance can be varied even after the inductor elements are assembled in circuits, have been proposed, e.g., in Japanese Patent Laying-Open Nos. 7-142258 and 8-162331.
For example, the inductance-variable element disclosed in Japanese Patent Laying-Open No. 7-142258 includes a spiral electrode formed on a semiconductor substrate with an insulating film therebetween and switch circuits for short-circuiting various turn portions of the spiral electrode.
In this structure, when the switch circuit is turned on in response to a predetermined applied voltage, the corresponding turn portion of the spiral electrode is locally short-circuited. This changes the number of turns of the inductance-variable element so that the inductance-variable element changes its inductance as a whole.
As already described, oscillation frequency fosc in the conventional VCO is controlled by variable capacitance Cvar. However, the equivalent parallel resistance of the LC resonant circuit lowers with increase in variable capacitance Cvar. Therefore, VCO may deviate from an oscillation state if the capacitance value is high. Accordingly, it is difficult to achieve a wide oscillation frequency range.
Further, oscillation amplitude Aosc of the VCO is proportional to oscillation frequency fosc. In a low frequency range, therefore, oscillation amplitude Aosc is low, and a signal-to-noise ratio of the oscillation signal is low so that the phase noise characteristics are impaired.
The foregoing inductance-variable element suffers from such a problem that the Q value lowers due to an on-resistance of a switch circuit connected in series to the inductor element. This results in deterioration of the phase noise characteristics of the oscillator circuit formed of the inductor element.